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Complete Measurement of Helicity and its Dynamics in Vortex Tubes

Helicity is a measure of cork-screw-like motion described by the amount of twisting, writhing, and linking in a fluid. Total helicity is conserved for ideal fluids, but how helicity changes in real fluids with even tiny amounts of viscosity has been an open question. Scheeler et al. provide a complete measurement of total helicity in a real fluid by using a set of hydrofoils to track linking, twisting, and writhing. They show that twisting dissipates total helicity, whereas writhing and linking conserve it. This provides a fundamental insight into tornadogenesis, atmospheric flows, and the formation of turbulence. reports on new findings in helicity dynamics:

University of Chicago physicists working in the nascent field of experimental vortex dynamics have, with unexpected help from a Sharpie marker, achieved the first measurements of an elusive but fundamental property of fluid flow.

Until now, there had been no way in the laboratory to measure the total helicity, or the measure of when two  rings intertwine. In their experiments, the UChicago team created thin-core vortices—the kinds found in aircraft wakes and insect flight—by producing hydrofoils using a 3-D printer.

As luck would have it, the red Sharpie marker used to label the hydrofoils contained rhodamine dye, which fluoresced when illuminated by laser light. When the hydrofoils were placed in a water tank, the dye began to diffuse, and when the hydrofoil was accelerated, the dye got sucked into the core of the newly created vortex—a process recorded via high-speed laser scanning tomography.

The new findings, published Aug. 3 in Science, are the first to show that helicity maintains a constant value during the flow of viscous fluids. Vortex dynamics have important applications in everyday life; meteorologists, for example, view helicity as a factor that contributes to the formation of supercell tornadoes.

"The fact that we have some measurements for the first time that show helicity can be preserved, especially in the presence of stretching, can translate directly to those efforts," said William Irvine, an associate professor in physics, who published the findings along with four co-authors.

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